The present invention relates to a mixing valve of the type employed where a plurality of liquids are to be mixed at very precise ratios, i.e. in liquid chromatography for generating a solvent gradient.
The process of mixing solvents is very common in liquid chromatography and is usually referred to as low-pressure gradient mixing. According to this process, small quantities of different solvents are drawn in succession by means of a pump forming part of the chromatograph and transferred in the mixed condition. The mixing valve is located between the supply tanks and the pump, and only one of the supply tanks is connected at any time with the pump. Since it is necessary to adhere very closely to the prescribed mixing rates in order to achieve high chromatographic measuring accuracy, very stringent demands are placed on the valve.
There have been mixing valves consisting of four valve units each having one inlet for the solvent to be drawn in and one closing element. A common discharge line is provided for the four valve units. In the case of these valves, liquid is drawn in at any time from one of the four supply tanks in response to activation of the respective closing element by respective lifting magnets. The closing element of known mixing valves comprises a conical element made of Teflon brand flouroplastic and a matching conical valve seat which is also made of Teflon in order to achieve the best possible sealing effect. For closing and opening a valve unit its conical teflon element is pushed into or retracted from the valve seat by means of an actuating element. However, the cooperating sealing elements of the known mixing valve tend to deform in the course of their service life so that the switching times required for completely closing or opening the valve increase gradually. Finally, the switching times increase in such a manner that the mixing valve no longer operates satisfactorily and has to be replaced by a new valve.